Basic Concepts in Soil Fertility

Jonathan DeenikAssistant Specialist, Soil Fertility

Department of Tropical Plant and Soil Sciences

Soil Fertility WorkshopFebruary 23 & 24, 2005

Outline• Definition and Significance• Historical Perspectives• Soil as a Nutrient Reservoir• Organic Matter• Soil Reaction• N and P• Cations• Micronutrients

Definition“The status of a soil with respect to its ability to supply elements essential for plant growth without a toxic concentration of any element”

Foth & Ellis, 1997

Historical Perspectives

Neolithic

10,000 yrs ago

Medieval

Up to16 thCent

Rotations

Justus von Liebig/Morrill Act

1862

Law of the MinimumMorrill Act

Ancient Greece and Rome

2,000 yrs ago

IrrigationManureLime

http://www.nri.org/InTheField/bolivia_s_b.htm

http://www.classicadventures.com/pages/itin_bike_tour_greece.html

Soil Plant Relationships

Havlin et al., 2005. Soil Fertility and Fertilizers

Soil as a Nutrient ReservoirClays

Properties:

• Non-expanding• pH dependent charge• low CEC (1-10 cmolc kg- 1)• Relatively low surface area• Non-sticky

Kaolinite

http://soil.gsfc.nasa.gov/forengeo/thnsec.htm

Soil as a Nutrient ReservoirClays

Montmorillonite

Properties:

• Expanding• Constant charge (-)• High CEC (80-120 cmolc kg- 1)• High surface area• Sticky

http://webmineral.com/specimens/picshow.php?id=1285

Surface ChargeConstant Charge• Net Negative• Isomorphic substitution

Brady & Weil, 2004. Elements of the Nature and Properties of Soils

Surface ChargepH-Dependent Charge• Net Negative or net positive• No isomorphic substitution• pH-dependent charge associatedwith broken edges of kaolinite and surfacesof gibbsite, geothite, hematite, allophane, and organic matter

Protonation De-protonation

Buffering CapacityThe ability of the soil to resupply an ion to the soil solution

Nutrientin

solution

Plant uptake

Solid phase nutrient(adsorbed/absorbed)

Total soil nutrient(unavailable)

Buffering capacity depends on:• clay content and type• CEC• organic matter

Organic Matter

HumusBrady & Weil, 2004. Elements of the Nature and Properties of Soils

http://www.sct.embrapa.br/diacampo/2004/releases.htm

Organic Matter

Composed of complex organic compounds: Humic and Fulvic acids and humin (60-90% of soil) Derived from OM deposited centuries agoResistant to microbial action

Functions:Water sorptionCECGlue

Composed of less complex non-humic substances: polysaccharides (10-30% of soil)Microbially modified and synthesized compoundsDerived from recently deposited OM (1-2 yrs)Susceptible to microbial action

Functions:Source of energy for microorganismsAggregationSource of mineralizable N, P and S

Stable OM Active OM

Soil is Alive

Source: Thompson and Troeh, 1978

Importance of Soil Biology- diversity- nutrient cycling- pest/pathogen suppression

- symbioses

Management Affects SOM

Tillage increases OM decompositionSOM regeneration depends on organic inputsSOM accumulation slow because most of the inputs released as CO2 http://www.gov.mb.ca/agriculture/soilwater/soil/fbe01s09.html

Soil Acidity

Natural Sources of Acidity:

Carbonic acid and organic acidsOrganic matterPrecipitation and cation leachingNitrificationN ImmobilizationAmmonium volatilizationCation uptakeDeprotonation of pH-dependent charge

Human Induced Acidity:

Acid rainUreaAmmonium fertilizersMono and diammonium phosphateElemental S

Soil Acidity40 years of N application

Source: Schwab et al., 1990 SSSAJ

(NH2)2CO + 4O2 2NO3- + 2H+ + CO2 + H2O

Adverse Effects of Soil AcidityAluminum toxicityManganese toxicityNutrient deficienciesDecreased microbial activity

Brady & Weil, 2004. Elements of the Nature and Properties of Soils

Havlin et al., 2005. Soil Fertility and Fertilizers

Acidity in Hawaiian Soils

Al toxicity uncommon - Kaolinite and Al/Fe oxides stable- pH in soils with variable charge minerals moves

toward pH0- High OM levels complex Al

• Mn toxicity more common as soil pH approaches 5.0

• Ca and P deficiency widespread in weathered oxidic and andic soils

Correcting Soil Acidity is Costly

Brady & Weil, 2004. Elements of the Nature and Properties of Soils

Correcting Soil Acidity is Costly

Al3+ + H2O Al(OH)2+ + H+

Liming Reactions:

CaCO3 + H2O Ca2+ + HCO3- + OH-

OH- + H+ H2O

HCO3- + H+ H2CO3

Al3+ + 3OH- Al(OH)3

Water1. Nutrients move to the

roots with water2. Leaching

- Nitrate, - cations

3. Saturated soils- Loss of N

(denitrification)- Accumulation of toxic

compounds (H2S, CH4)4. Salinity

- salt build-up under dry conditions

Nitrogen

http://www.bettersoils.com.au/module2/images/27.gif

N Mineralization

Mineralization: Decomposition of soil organic matter by soil microbes releasing inorganic N in the process.

Heterotrophs use organic molecules as source of energy• Bacteria – neutral to alkaline environments• Fungi - acidic environments

Release of N from the organic matter• Soil Organic Matter ~5% N• 1 to 4% organic N mineralized each year• Added organic N sources

(C:N ratio < 20 = mineralization

ImmobilizationImmobilization• Conversion of mineral N to organic N by microbes

Organisms that decompose organic matter as an energy source require nitrogen

Organic materials with a low N content (C:N > 30) cannot supply the needs of these organisms thus they use soil N in competition with the crop.

Freshly immobilized N = 5-15% of soil N

Ammonium N

Ammonium N = NH4+

• Cation, therefore adsorbed on CEC

• Won't leach or denitrify

• Can be fixed in certain clay minerals – micaceous clay

• Plant uptake

• Very common source of N

• Rapidly converted to NO3-N under most conditions

• Volatilization at high pH

NH4+ + OH- → NH3↑ + H2O

High pH Gas

Nitrate N• Anion, therefore not adsorbed on CEC• Most common mineral form of N in most soils• Most common form taken up by plants• Very susceptible to leaching and denitrification

losses2NO3

- → N2O & N2 + 3O2Anaerobic gases

No oxygen - wet soil

Energy source for bacteria - organic matter

Warm temperatures

Favored by higher pH

Phosphorus

http://biology.kenyon.edu/courses/biol112/Biol112WebPage/Syllabus/Topics/Week%2013/PhosphorusCycle.jpg

Effect of pH on P Fixation

http://www.regional.org.au/au/asssi/supersoil2004/s11/oral/1840_harringtonb.htm

Factors Affecting P Fixation

Soil type

Andisol>Oxisol≈Ultisol>Inceptisol>Mollisol≈VertisolHonokaa Kapaa Kunia MakaweliWaimea Wahiawa Waialua Kula Alaeloa Keahua

Kahana Lualualei

Organic matter- OM imparts negative charge to surfaces inhibiting reaction between phosphate and oxide surface

Base CationsCa++, Mg++, K+, Na+

Means of expression:- extractable (mg kg-1, ppm)

in solution + on exchange site- exchangeable (cmolc kg-1, meq/100g)

on exchange siteBase Saturation (BS)

%BS = (∑Exch bases/ CEC)x100% Ca Sat = (Caexch/CEC)x100Converting ppm to cmolc kg-1

Cappm/200, Mgppm/120, Kppm/390

Calcium AvailabilityFactors:- total Ca supply & pH- CEC, clay mineralogy- %Ca++ sat- ratio of Ca++ to other cations

General Rules:- 15 ppm Casol sufficient- Most crops respond to Ca when Casat < 25%- 2:1 clays require >70% Casat

- 1:1 clays 40-50% Casat suff.

Brady & Weil, 2004. Elements of the Nature and Properties of Soils

Calcium Availability

Example:Caextr = 1000 ppm or Caexch = 5 cmolc kg-1

Oxidic Soil: Smectic Soil:CEC = 9 cmolc kg-1 CEC = 34.1 cmolc kg-1

Casat = 5/9x100 = 55.6% Casat = 5/34.1x100 = 14.7%

In oxidic soil 1000 ppm likely adequate, but smectitic soil isCa deficient. If we want 70% Casat we must add 18.9 cmol kg-1 Ca (6,733 lbs Ca/acre).

If we assume no response after 25% Casat, then we should add 1,257 lbs Ca/acre.

Magnesium Availability

Factors:- total Mg supply & pH- Al saturation- ratio of Mg++ to other cationsGeneral Rules:- Mgsat 4 - 20%- Mgsat 7-10% ideal (mainland textbook)

Brady & Weil, 2004. Elements of the Nature and Properties of Soils

Potassium AvailabilityFactors:- total K supply & pH- texture: fine > coarse- CEC- type of clayGeneral Rules:- solution K 1 -10 ppm- K uptake influenced by exchangeable Ca and Mg

Cation Ratios

Concept of fertilizing to achieve balanced Ca:Mg ratio (6.5:1) developed by Bear and co-workers (1940’s). W. Albrecht adopted the approach and claimed:

- Reduced weed populations- Stimulated microorganisms- Better “balance” of soil nutrients”- Improved plant health

Private labs (i.e. Brookside) use ratios, University labs use sufficiency approach

Cation Ratios

Mclean and co-workers (1983) conduct 6 year study and conclude no specific ratio necessary, but avoid extremes (too wide, too narrow)Range of cation ratios

- Ca:Mg: 2:1 to 26:1 no effect on temperate climate yields, ratios < 1 lead to problems

- Mg:K: suggested around 2.5-5, when < 1 Mg deficiency likely

MicronutrientsResearch limited in Hawaii

- Zn in coffee production (Hue et al., 2004)- B requirement for avocado (Miyasaka, 1999)- OM effects on micronutrient forms (Li et al., 1997)

- Pot studies on Zn response in Paaloa,Keahuasubsoils (Rashid & Fox, 1992)

- B requirements of macadamia seedlings (Fox, 1989)

- B response in Hawaii soils (Hue et al., 1988)

- Zn response in sugarcane (Huang, 1974)

Manganese

Mn2+ form of plant uptake- low solubility as pH increases

Mn toxicity can be a serious problem in Hawaii soils - especially weathered Oxisols/UltisolsFactors:

1. Low pH (<5.5)2. High soil moisture (waterlogged)3. Increased availability with OM in mineral soils

Management:- maintain soil pH above 5.5

Mn deficiency can occur in high pH soils

Iron

Fe2+ taken up by plantsSolution Fe low in soils- Fe forms chelates with OM increasing

phytoavailability- Fe commonly deficient in calcareous soils- Fe deficiency can occur in acid soils with

high MnFe deficiency often corrected with foliar application of Fe.

Zinc

Zn solubility low in soilsForms chelates with OM to increase phytoavailability in mineral soils, but can lead to deficiency in organic soilsZn uptake reduced in when other metal cation concentrations are high, also high P can induce Zn deficiency in marginally deficient soilsZn deficiency most common in high pH soilsZn fertilizer broadcast/banded in field crops and foliar applied for orchards and vegetables

Copper

Cu solubility low in soilsForms chelates with OM to increase phytoavailability in mineral soils, but can lead to deficiency in organic soilsCu deficiency more common in organic soils and coarsely textured leached soilsCu deficiency most common in high pH soils

Boron

Narrow range in soils separating deficiency from toxicityB deficiency common in high pH soils and in dry soil conditionsOM increases B availabilityCelery, broccoli, cauliflower highly sensitive to B deficiencyFoliar applications common at 0.09 to 0.4 lbs/a

Resources

BooksHavlin, J.L., S.L. Tisdale, J.D. Beaton, and W.L. Nelson. 2005. Soil Fertility and Fertilizers. Pearson Education, Inc., Upper Saddle River, NJ

Foth, H. D. and B.G. Ellis. 1997. Soil Fertility. CRC Press, Inc., Boca Raton FL.Brady, N.C. and R.R. Weil. 2004. Elements of the Nature and Properties of Soils. Pearson Education, Inc., Upper Saddle River, NJ

Webhttp://www.extension.iastate.edu/pubs/so.htmhttp://www.montana.edu/wwwpb/pubs/mt4449.htmlExcellent short course in soil fertility